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The combined measles, mumps and rubella, or MMR, vaccine is offered in the U.K. in two doses to all children, first at 12 months and then again at 3 years. Vaccination rates have dropped to about 85% nationally, and far lower in parts of London, according to U.K. Health Security Agency chief executive Jenny Harries.

That is “too low to maintain safe population coverage — we want that at about 95%" as advised by the World Health Organization, she said.

Public health officials say more than 3.4 million children under 16 years old are unprotected and at risk of catching the preventable diseases.

Measles was declared eliminated in the U.K. in 2017, meaning the disease was no longer native to the country.

But transmissions began again when epidemics broke out in the rest of Europe. Officials said outbreaks can take place anywhere where vaccine coverage is below the 95% needed to achieve herd immunity.

WHO and the U.S. Centers for Disease Control and Prevention said in November that spiked by more than 40% last year, and cases rose after vaccination levels dramatically dropped during the COVID-19 pandemic.

Measles is among the most infectious diseases known and spreads in the air when an infected person coughs or sneezes. It's most common in children under 5. Symptoms include fever, cough, runny nose and a distinctive rash.

Infection can lead to permanent physical damage such as deafness. Most deaths are due to complications like encephalitis, severe dehydration, serious breathing problems and pneumonia.

Harries said there was “clearly misinformation� about vaccines and urged people to consult trusted sources.

Officials said intense media coverage in the late 1990s about the now discredited claim linking the MMR vaccine to autism led to a drop in vaccinations and took many years to recover from.

More recently, declines in vaccination uptake were exacerbated partly by the pandemic and anti-vaccine sentiment, said Helen Bedford, a professor of children's health at University College London.

“The pandemic, with new vaccines introduced and vaccination constantly discussed, may have resulted in the public having more questions about vaccination: its safety, effectiveness and, for a disease like measles which had become rare in the U.K., its necessity,� she said. “Asking questions about vaccination is to be encouraged, but we need trained staff to do this.�